1. Field of the Invention
This invention relates to control units used to automatically turn on room lights, ventilating fans, or heating/cooling equipment whenever the room is occupied. More particularly, the invention relates to control units responsive to occupancy sensors which are capable of switching modern high efficiency fluorescent lights and similar loads.
2. Description of Related Art
Occupancy sensing switch control units are used to reduce energy costs by automatically turning on room lights or environmental control equipment when the room is entered, and turning off the equipment when the room is unoccupied.
Previous control units have used a mechanical relay to carry the AC current load of the lighting. The relay is controlled by an occupancy sensor and associated circuitry to drive the relay actuator. The sensor provides a control signal which operates the relay and turns the lights on whenever the room is occupied.
The occupancy sensor has most commonly been an infrared or ultrasonic detector. The associated circuitry has included a timer which is set each time the control signal indicates that the room is occupied. While the timer is set, and during its timer period, the lights are on. When the timer period ends, the lights are turned off. Small movements by anyone in the room continuously resets the timer to keep the lights on. In the absence of the continual resets, when the room is unoccupied, the timer reaches the end of its timer period and automatically shuts off the lights or other equipment.
This design has worked well to switch standard incandescent and fluorescent lighting. However, such standard lighting fixtures are less efficient than modern high efficiency lights which use electronic ballasting. Electronic ballast places a much different load upon the control unit than standard lights.
It has been found that prior art control units fail prematurely when they are used with electronically ballasted high efficiency fluorescent lights. Although electronically ballasted lights are presently used in only a small percentage of lighting installations, they are likely to be specified for use whenever energy efficiency is a particular concern. Thus, there is a clear need for an occupancy sensor based lighting control unit which can be used in combination with high efficiency lighting.
One difficulty with AC powered high efficiency fluorescent lighting is that it presents a very noisy load for the control unit to switch. Such lights also have a high initial inrush current which must be handled by the control unit switching system. The noise generated when turning on and turning off of the lights has a deleterious effect on the control unit circuitry, and the inrush current can damage the contacts of a mechanical relay which would otherwise easily be capable of handling the steady state load current of the light.
Accordingly, one object of the present invention is to provide an occupancy sensor based control unit which has a long life when used with AC powered high efficiency lighting.
Another object of the present invention is to provide an occupancy sensor based control unit which operates reliably to switch a noisy electrical load and operates reliably in a noisy electrical environment.
Yet another object of the present invention is to provide a control unit switch which can handle high initial inrush currents.